The subject invention pertains to drying paper webs to which a liquid coating has been applied and, more particularly, to drying aqueous-based zone coatings applied to a traveling paper web.
Paper webs are treated with a wide variety of liquid coatings for various purposes. The coatings may be aqueous-based or utilize some other liquid base. The coatings may be applied to provide a protective layer, an adhesive layer, a printed indicia, or a decorative coating. When the coatings are applied in zones, i.e. covering less than the entire face of the web, there is inevitably a differential penetration of the liquid coating vehicle into the coated portions of the web as opposed to the portions which are not coated. With a water-based coating vehicle, moisture penetration into the paper causes the fibers to swell and the resultant expansion often causes wrinkling of the web. Subsequent drying of the coated web may leave permanent wrinkles or result in curling of the paper products formed from the web. For example, in the production of corrugated paperboard in which two or more paper webs are glued together with a water base starch adhesive, the ever-present problem of moisture control is acutely apparent in the zone coated adhesive which is selectively applied to the flute tips of the corrugated medium and similarly transferred to the liner web with which the medium is combined. Even in the coating of a unitary web, for example applying continuous longitudinally extending, laterally spaced zone coats of an aqueous based coating material to a running paper web, moisture penetration into the paper often results in differential expansion and permanent wrinkling which remains when the web is subsequently dried.
In the manufacture of single face corrugated paperboard, in addition to problems directly associated with moisture variations and differential expansion, it has always been a challenge in the prior art to effect an adequate bond without crushing or unacceptably distorting the flutes of the corrugated medium. The water-based starch adhesive typically used in corrugating is a thermosetting adhesive requiring relatively high temperatures, in the range of 180.degree.-200.degree. F. (82.degree.-93.degree. C.), to cause gelatinization of the starch adhesive. In a conventional single facer, the adhesive is applied by a glue roll to the flute tips on one face of the corrugated medium while the medium is still on one of a pair of corrugating rolls. The liner web is immediately thereafter brought into contact with the coated flute tips by a pressure roller which holds the medium against the flute tips supported from behind by the corrugating roll. As indicated, the liner web, as well as the corrugated medium, are preheated and, in combination with the pressure provided by the pressure roll, causes gelatinization and curing of the adhesive.
Operative contact between the pressure roll and the corrugating roll (with the single face web running therebetween) results in vibration and noise as the pressure roller passes intermittently from tip to tip of the fluted corrugating roll. The problem is aggravated by the high pressure used to hold the liner web against the fluted medium in the nip between the pressure roll and the corrugating roll. Attempts have been made to eliminate this problem by utilizing stationary pressure members which have an arcuate surface corresponding generally to the flute tip diameter of the corrugating roll. Such stationary pressure members are shown, for example, in U.S. Pat. Nos. 4,337,884 and 4,481,066. A similar pressure member, but which includes a belt moving over the arcuate surface thereof, is shown in U.S. Pat. No. 4,316,761. Although the foregoing patents address the problem of vibration and noise, they still require a high nip pressure which can result in flute damage and, furthermore, requires significantly more power to pull the web through the single facer. Such a single face web drive is shown for example in the above identified U.S. Pat. No. 4,481,066, which utilizes a vacuum-assisted drive belt to pull the single face web through the single facer. It is also known to use a pair of opposed belt conveyors to capture the glued single face web therebetween to pull the web through the single facer. The latter apparatus, however, may subject the fluted medium to undesirable crushing loads.
Thus, prior art single facers typically utilize means to preheat the component webs to a relatively high temperature, and a high pressure nip means to effect the starch-based adhesive bond between the medium and the liner. Once the single face web leaves the nip between the corrugating roll and the pressure roll or pressure member, treatment of the web is completed and, if the web is to be used to make a double face corrugated web, the single face web is directed into an accumulating bridge storage device, as is well known in the art.